Detailed Geologic Mapping of the Central Brooks Range Foothills: Sagavanirktok River to Nanushuk River

By

E.E. Harris (Alaska Division of Geological & Geophysical Surveys), C.G. Mull (Alaska Division of Oil & Gas), R.R. Reifenstuhl, D.L. LePain, and P.R. Peapples (Alaska Division of Geological & Geophysical Surveys)

Detailed mapping (1:63,360-scale) in the area of Atigun Gorge to Slope Mountain and west to the Cobblestone Creek area in the central Brooks Range foothills reveals much about its depositional and structural evolution. Carboniferous to Lower Cretaceous strata at the mountain front form the Endicott Mountains allochthon. The earliest Cretaceous (Neocomian) Okpikruak Formation, which records foreland basin fill into the Colville basin during early-stage Brooks Range orogeny, was subsequently thrust northward on the Ipnavik River and Picnic Creek allochthons during the Aptian. Mid- Aptian to Albian debris-flows and turbidites derived from the allochthons formed the Cobblestone Member and Fortress Mountain Formation, accompanied by probable syn-depositional, north-vergent thrusting. The Late Albian to Cenomanian was marked by prograding pro-delta shale of the Torok Formation, followed by deposition of the fluvial-deltaic Nanushuk Formation derived from unroofing of the Endicott Mountains allochthon during a second episode of Brooks Range orogeny. Late Cretaceous marine transgression with organic-rich shales of the Seabee Formation was followed by eastward prograding nonmarine and marine clastics of the Tuluvak Formation. Renewed thrusting during the early- to mid-Tertiary resulted in duplexing at the mountain front and re-thrusting and refolding of the Cobblestone parautochthonous belt and allochthonous rocks in the Atigun Gorge area. Regional shortening in the southern Colville basin folded the Nanushuk and overlying Upper Cretaceous rocks above a decollement in the Torok, with south-vergent backthrusting at a triangle zone between the Cobblestone parautochthonous sequence and the Tuktu Escarpment.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.